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Chiari Malformation: A Literature Review

Jessica Heithoff

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Abstract

This literature review was designed to address a pathology that has recently

become more commonly diagnosed among individuals with the help of Magnetic

Resonance Imaging. The purpose is to define and discuss information previously

introduced in peer-reviewed journal articles and textbooks for a thorough understanding

of Chiari Malformation. There are many types of Chiari Malformations. It is a congenital

condition characterized by the protrusion of brain tissues into the spinal canal through the

foramen magnum. The exact cause of the disease remains unknown. A few hypotheses

have been formed and are discussed in this paper. Diagnostic imaging techniques such as

Radiography, Computed Tomography, and Magnetic Resonance Imaging are used for the

diagnosis of Chiari Malformations. The symptoms that may or may not be present vary

among individuals. The treatment methods depend on the severity of the symptoms.

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The coordination of motion and balance of an individual is controlled by the

cerebellum. It is located inside the base of the skull in the cavity known as the posterior

fossa. The posterior fossa houses the cerebellum, brainstem, and cranial nerves 5-12.1

The cavity behind the brainstem and in front of the cerebellum that holds cerebral spinal

fluid (CSF) is known as the fourth ventricle. The cerebellum is composed of two lateral

halves or hemispheres. Two small protrusions called the cerebellar tonsils are located

underneath the cerebral hemispheres. All of these structures are located just above the

foramen magnum which is an oval-shaped opening at the base of the skull. The spinal

cord enters the skull through the foramen magnum and connects to the brainstem.

Various alterations to the posterior fossa, cerebellum and foramen magnum such as

Chiari Malformations have been noted throughout research with the help of diagnostic

imaging. A Chiari Malformation is characterized by an anatomic defect at the base of the

skull in which the cerebellum and brain stem herniate through the foramen magnum into

the cervical spinal canal (see Figure 1).2

Figure 1. A sagittal MRI image demonstrates the cerebellar herniation through

the foramen magnum at the base of the skull circled in yellow.3

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The material presented in this paper will give a better understanding regarding the

different classifications, etiology, clinical manifestations, diagnosis and treatment for

Chiari Malformations by analyzing previous research and studies. The best way to

diagnose Chiari Malformation is with the help of Magnetic Resonance Imagining (MRI),

so the information will include important specifics about the aspects of diagnostic

imaging.

Chiari Malformation is named after a German pathologist, Professor Hans Chiari.

In 1981, he classified the different malformations into types I, II, III and IV based on the

findings of autopsies he was performing.4 Julius Arnold made additional contributions to

Type II Chiari Malformation. Therefore, in his honor, Type II was later named Arnold-

Chiari Malformation.5 Future researchers did not classify later findings of Chiari

Malformation using Roman Numerals. Instead, Type 0 and 1.5 were added to Chiari’s

original four.

Classification

Chiari Malformation Type I is the most common type generally diagnosed in

adolescents or adults. It occurs during fetal development and is characterized by the

downward displacement of the cerebellar tonsils beneath the foramen magnum by at least

3-5 millimeters.1,4 The displacement can block the normal CSF flow between the

intracranial space and the spinal cord. Anomalies where the cervical area of the spine

compresses into the base of the skull resulting in brainstem compression is noted in 30-

50% of patients with Chiari Malformation Type I. Lateral curvature of the spine, known

as Scoliosis, may be present. Spina Bifida is noticed in conjunction with Chiari

Malformation as well. This condition is the result of incomplete development of the

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spinal cord and its protective covering. Part of the spinal cord is exposed and may extend

out a patient’s back in a sac-like pouch. Syringomyelia may also be present (see Figure

2). Syringomyelia is a disorder where a CSF-filled tubular cyst, also known as a syrinx,

forms within the central spinal cord.6 As the syrinx grows it destroys the spinal cord

causing some of the symptoms of Chiari Malformation.

Figure 2. A sagittal MRI image where the blue arrow represents the Chiari

Malformation extending below the foramen magnum with little CSF flow. The red arrow

represents the syringomyelia.3

Chiari Malformation Type II becomes symptomatic in infancy or early childhood

as it is more severe than Type I. It is characterized by the caudal displacement of the

medulla, fourth ventricle and cerebellar tonsils into the cervical spinal canal. Chiari

Malformation Type II commonly occurs in children with Spina Bifida or

Myelomeningocele. Myelomeningocele is a congenital condition in which the spinal cord

and column do not close properly during fetal development. Because of this, the spinal

cord and the protective membrane may protrude similar to Spina Bifida. The majority of

patients with Chiari Malformation Type II also have hydrocephalus, an excessive buildup

of CSF in the brain. Chiari Malformations often block the flow of CSF between the skull

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and spine. Therefore, pressure builds up in the brain causing an abnormally large head or

mental defects. A flexible tube called a shunt may need to be placed from the head to

under the skin in the chest or abdomen to divert and drain the excess fluid to be absorbed

by the body.6,7

Chiari Malformation Type III is very rare. This type is characterized by a portion

of the cerebellum and brainstem pushing out through an occipital encephalocele. An

encephalocele is an abnormal opening in the back of the skull (see Figure 3).6 This leads

to a bony defect similar to spina bifida. The meninges or brain tissue herniates outside the

skull. Chiari Malformation Type III has a high and early mortality rate or severe

neurological deficits in surviving patients.

Figure 3. This axial MRI image demonstrates an occipital encephalocele protruding out

the back of the skull circled in yellow.8

Chiari Malformation Type IV is the most severe and rarest form with an

extremely high mortality rate in infancy. The cerebellum does not develop normally

during pregnancy. Other malformations of the brainstem may also be present.

Chiari Malformation Type 0 is characterized by an alteration in the cerebrospinal

fluid hydrodynamics at the level of the foramen magnum not necessarily with cerebellar

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tonsil herniation.2,4 Patients with this type of Chiari Malformation often develop

syringomyelia. Some physicians do not consider Type 0 to be a Chiari Malformation

because there is no cerebellar tonsil herniation.9

Chiari Malformation Type 1.5 is just a more advanced form of Type I. The entire

cervicomedullary junction protrudes through the foramen magnum, not just the cerebellar

tonsils.10 Diagnosis of Chiari Malformation Type 1.5 are rare because some doctors are

not familiar with this type.

Etiology

Chiari Malformations are due to the developmental failure of the brainstem,

cervical region of the spine and base of the skull including the foramen magnum. The

exact cause of the developmental failures leading to Chiari Malformations is unknown.

Chiari Malformations affect individuals of every race and ethnicity, but some studies

suggest that females are more often affected versus males. The abnormally small

posterior fossa may lead to the growing brain to be pushed down into the foramen

magnum. Exposure to harmful substances or not enough vitamins and nutrients during

fetal development, post-natal osseous growth disturbances, or a genetic association are

hypothesized causes of the the small posterior fossa.6,11,12 A number of studies suggest

that genetic factors play an important role in Chiari Malformations. Research has been

conducted on familial aggregation and a higher degree of concordance in monozygotic

twins.12 To date, no single gene has been linked to Chiari Malformations. A study was

conducted sampling 415 patients with Chiari Malformation symptoms. A diagnosis was

made based on cerebellar tonsil herniation of greater than 3 mm on these participants.

The control sample was composed of 524 healthy blood donors that did not have Chiari

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Malformation.12 58 developmental genes were tested, but more confirmatory studies are

needed as this was one of the first studies of its kind.

Another study was conducted using MRI images from 23 families with 71

affected individuals. The heritability and cranial morphology correlations were measured

among the individuals.13 A linkage of over 10,000 single-nucleotide polymorphisms

(SNPs) across the genome were used to identify regions of linkage to Chiari

Malformation. A SNP is the most common variation in the human genome and occurs

approximately once every 100 to 300 bases.14 The study suggested linkage to

Chromosome 9 and 15.

Based on the results of the studies, it is suggested that Chiari Malformations

originates as a disorder of para-axial mesoderm.5,12 This is consistent with the formation

of a small posterior fossa. It is possible to acquire Chiari Malformation instead of being

born with the condition. Any circumstance in which space is occupied in the posterior

fossa such as a tumor, arachnoid cyst, hematoma or hydrocephalus may cause a Chiari

Malformation to develop.9

Clinical Manifestations

The signs and symptoms of Chiari Malformation vary from person to person.

Some individuals may be asymptomatic with Chiari Malformation being an incidental

finding. The symptoms of Chiari Malformations involve various areas of the central

nervous system. The symptoms have been grouped into five syndroms: increased

intracranial pressure, involvement of the cranial nerves, brainstem compression, spinal

cord syndrome and cerebellar syndrome (see Table 1) .15

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Table 1. Signs and symptoms related to Adult Chiari Malformation.15

System Subjective Symptoms Objective Signs

Ocular

Transient visual

obscurations, photophobia,

diplopia, retro-orbital

pressure, visual field

abnormality

Papilledema, absent venous

pulsations, decreased

acuity, extraocular muscle

palsy

Otological

Dizziness, tinnitus,

decrease hearing, ear

pressure, vertigo,

hyperacusis

Nystagmus sensorineural

hearing loss, abnormal

vestibular testing

Lower Brainstem, lower

cranial nerves

Dysphagia, dysarthria,

sleep apnea, throat pain,

palpations, syncope,

shortness of breath,

hypertension

Impaired gag reflex, vocal

cord paralysis, hypoglossal

nerve palsy, spinal nerve

palsy

Cerebellar

Unsteady gait, poor

coordination, impaired fine

motor function, tremor

Dysmetria, ataxia

Sensory systems

Retro-orbital and occipital

headaches, cervical pain,

facial and acral numbness,

paresthesia and pain, poor

position sense, burning

dysesthesia

Analgesia, impaired

proprioception

Motor systems

Weakness

Weakness, spasticity,

hyperreflexia

Other

Chronic fatigue, altered

recent memory, nausea,

vomiting, incontinence,

impotence, trophic

disturbances

Headaches are seen in 80% of patients that are experienced occipitally and may

radiate into the neck and shoulders. The symptoms may worsen with exertion such as

coughing, sneezing, straining or neck extension. Syringomyelia, often in conjunction with

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Chiari Malformations, is associated with a variety of symptoms. The symptoms depend

on the size and location of the syrinx and include loss of muscle mass, weakness,

numbness, chronic pain or scoliosis.9

Diagnosis

The diagnosis of Chiari Malformations is related to the patient’s symptoms. Some

patients experience symptoms specific of Chiari Malformation while others experience a

gradual onset of symptoms that are often misdiagnosed with another condition. Some

patients do not experience any symptoms at all. In this case the finding is incidental as

the patient had neuroimaging for another reason. Adult patients often go 5 years before

being properly diagnosed with Chiari Malformation.15,16

The diagnosis of Chiari Malformations is established with diagnostic imaging.

The diagnostic imaging modality of choice is MRI. In 1985, MRI became widely

available in the clinical practice.2 MRI is the use of magnetic radiofrequency pulses to

create 2D images. Images in the sagittal plane, which divides the body into left and right

halves, allows for the determination of the cerebral tonsil herniation (see Figure 4).

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Figure 4. A sagittal T1-weighted MRI image of the brain. The yellow line represents the

lower limit of the posterior cranial fossa. It is the reference point for measuring the

cerebellar tonsil herniation.17

MRI can also confirm compression of the posterior fossa subarachnoid spaces,

overcrowding in the posterior fossa, peg-shaped tonsils, a small posterior fossa, increased

slope of the tentorium, medullary kinking and basilar impression.15 Both brain and

cervical spine scans should be performed to rule out any associated intracranial anomalies

like hydrocephalus and syringomyelia (see Figure 5). Contrast should be utilized for the

scans to help validate any abnormal pathology, but especially in the case of a syrinx.

Contrast allows for visualization of the flow of CSF. It can be beneficial to investigate if

the Chiari Malformation is blocking the flow of the CSF between the brain and spine.16

Figure 5. An axial MRI of a child with hydrocephalus on the left and a normal brain for

comparison on the right. The ventricles are noted by the white arrows. The brain with

hydrocephalus demonstrates enlarged ventricles compared to the normal brain.18

Other diagnostic imaging modalities that may be utilized include Diagnostic

Radiography and Computed Tomography (CT). Both of these modalities utilize radiation

to create their images. Skull and cervical spine radiographs have little diagnostic value

when it comes to Chiari Malformations. Radiographs are only able to display boney

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abnormalities. CT is able to obtain thin slices through the spine and skull to be

reconstructed into the three planes, coronal, axial and sagittal. The thin slices can better

illustrate boney abnormalities.

Treatment

Chiari Malformation treatment is dependent upon the individual’s symptoms.

Patients with Chiari Malformation that do not experience symptoms are often not treated

at all. Conservative treatments such as pain medications, massage therapy or activity

reduction are usually suffice in individuals with mild symtoms.9 Patients with either no

symptoms or mild symptoms are checked regularly using MRI to monitor the progression

of the condition.

Patients that are very symptomatic are treated with surgery to ease and stabilize

their symptoms. Fifty percent or less of people diagnosed with Chiari Malformation

actually undergo surgery.19 The goal of the surgery is to stop the progression of changes

in the anatomy of the brain and spinal cord.7 Posterior fossa decompression is the

operation of choice. It is performed by neurosurgeons to create more room at the base of

the skull to restore CSF flow between the brain and spinal canal. There are many steps

involved. The first thing that will happen in surgery is a craniotomy, when a piece of the

skull is removed. A laminectomy may be performed to remove one or more of the

vertebrae. A duraplasty is when the tough outer covering of the brain is cut open and a

patch is sewn in to make it bigger.9,16 The cerebellar tonsils are sometimes shrunk down

by cauterizing. Mesh or an artificial plate is then inserted where the craniotomy took

place (see Figure 6). A typical surgical experience includes a 4 hour surgery, one night in

the ICU, 3-4 days in the hospital and rest at home for 3 weeks to 3 months.16

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Figure 6. This image illustrates the steps of the posterior fossa decompression surgery.20

Conclusion

Chiari Malformation findings are becoming more common as the use of

diagnostic imaging has increased. Sagittal MRI images are used to help diagnose and

classify the different types of Chiari Malformations. Researchers are working to narrow

down the genetic aspects that could possibly be the cause of this condition. The

symptoms of Chiari Malformations depend on which type and vary from person to

person. An occipital headache is the most common symptom among patients with Chiari

Malformation. Some patients do not experience any symptoms at all, while others range

from mild to severe. Conservative treatments should be considered for patients with mild

symptoms. Neurosurgeons use the MRI findings along with careful consideration to help

determine if a patient is a candidate for surgical intervention. The benefits of surgery

should always outweigh the risks. The goal of treatment is to allow adult and adolescent

patients with Chiari Malformations to continue to carry out activities in their normal

everyday life, or extend the life of an infant or child.

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